1. Field of the Invention
The present invention relates to a fuel-injection control apparatus for an engine, especially to a fuel-injection control apparatus for an engine having an air-fuel ratio learning control system.
2. Description of the Related Arts
An automobile is generally provided with a fuel-injection control apparatus which purges fuel-vapor to the inlet pipe and uses it as fuel in order to avoid air pollution.
In this apparatus, because use of the fuel-vapor disturbs the air-fuel ratio, purging is executed when the air-fuel ratio is controlled by the air-fuel ratio feedback control system.
Furthermore, because it is necessary to avoid an overflow from a charcoal canister which absorbs fuel-vapor evaporating from a fuel tank, it is desirable to dispose of fuel-vapor as much as possible by increasing the frequency, the period and the amount of purging.
In the air-fuel ratio feedback control system, the air-fuel ratio learning control system is generally applied in order to make the deviation of the air-fuel ratio from the stoichiometric air-fuel ratio small by gradually renewing an air-fuel ratio feedback correction factor (FAF). This system compensates for fluctuations in those factors, such as the error and the deterioration with the passage of time of the air-flow meter, the injectors, the pressure regulator and the control unit, the non-linearity of injectors, and changes in the driving condition, which influence the air-fuel ratio.
Both the air-fuel ratio learning control and the purge control are executed while the air-fuel ratio is controlled by the air-fuel ratio feedback control system, but the air-fuel learning factor renewed in the air-fuel learning control system cannot be used for an accurate air-fuel ratio control because it is calculated from the air-fuel ratio feedback correction factor influenced by fuel-vapor.
Therefore, it is difficult to execute these two controls simultaneously. If it is required that they be executed simultaneously, it is necessary that the executing time of the air-fuel ratio learning control be shortened and the purge control be executed as often as possible in order to learn effectively and increase the purge amount.
The air-fuel ratio control system which inhibits the air-fuel ratio feedback control until the engine is warmed-up, that is, until a predetermined time elapses after the engine has started, has been proposed because the burning in the cylinder is not stable due to insufficient atomization and the emission and/or the drivability are deteriorated when the engine is not warmed-up enough (refer to Unexamined Patent Application (Kokai) No. 59-176444).
If the air-fuel ratio learning control is inhibited until a predetermined time elapses after the engine has started, the frequency of the air-fuel ratio learning control is decreased. The start of the purging is delayed and the frequency of purging cannot be secured because the air-fuel ratio learning factor does not converge enough due to the transient operating condition if the air-fuel ratio learning control is begun after a predetermined time elapses.
Furthermore, if it is required that the air-fuel ratio feedback control is executed when the engine is not warmed-up enough, it is common that the air-fuel ratio learning control is inhibited in order to avoid faulty learning because the air-fuel ratio correction factor is influenced by the start enrichment and the warm-up enrichment.
Namely, the air-fuel ratio learning factor is influenced not only by the start enrichment and the warm-up enrichment, but also by the delay time which is inherently required until the air-fuel ratio sensor becomes active. Therefore, the air-fuel ratio learning control cannot substantially be executed when the engine is not warmed-up enough.
Furthermore, because the fuel amount of the warm-up enrichment is not constant and is varied in accordance with the degree of warm-up, the air-fuel ratio learning during warm-up is more difficult to perform.